2.1 Pathobiology of MDS
Myelodysplastic syndrome (MDS) refers to a diverse group of hematopoietic stem cell disorders. MDS is characterized by a cellular marrow with impaired morphology and maturation (dysmyelopoiesis), peripheral blood cytopenias, and a variable risk of progression to acute leukemia, resulting from ineffective blood cell production. The Merck Manual 953 (17th ed. 1999) and List et al., 1990, J. Clin. Oncol. 8:1424.
The initial hematopoietic stem cell injury can be from causes such as, but not limited to, cytotoxic chemotherapy, radiation, virus, chemical exposure, and genetic predisposition. A clonal mutation predominates over bone marrow, suppressing healthy stem cells. In the early stages of MDS, the main cause of cytopenias is increased programmed cell death (apoptosis). As the disease progresses and converts into leukemia, gene mutation rarely occurs and a proliferation of leukemic cells overwhelms the healthy marrow. The disease course differs, with some cases behaving as an indolent disease and others behaving aggressively with a very short clinical course that converts into an acute form of leukemia.
The actual incidence of MDS in the U.S. is unknown. MDS was first considered a distinct disease in 1976, and occurrence was estimated at 1500 new cases every year. At that time, only patients with less than five percent blasts were considered to have this disorder. Statistics from 1999 estimated 13,000 new cases per year, and about 1000 cases per year are reported in children, surpassing chronic lymphocytic leukemia as the most common form of leukemia in the western hemisphere. The perception that the incidence is increasing may be due to improvements in recognition and criteria for diagnosis. The disease is found worldwide.
An international group of hematologists, the French-American-British (FAB) Cooperative Group, classified MDS disorders into five subgroups, differentiating them from acute myeloid leukemia. The Merck Manual 954 (17th ed. 1999); Bennett J. M., et al., Ann. Intern. Med. 1985 October, 103(4): 620-5; and Besa E. C., Med. Clin. North Am. 1992 May, 76(3): 599-617. An underlying trilineage dysplastic change in the bone marrow cells of the patients is found in all subtypes.
There are two subgroups of refractory anemia characterized by five percent or less myeloblasts in bone marrow: (1) refractory anemia (RA) and; (2) RA with ringed sideroblasts (RARS), defined morphologically as having 15% erythroid cells with abnormal ringed sideroblasts, reflecting an abnormal iron accumulation in the mitochondria. Both have a prolonged clinical course and low incidence of progression to acute leukemia. Besa E. C., Med. Clin. North Am. 1992 May, 76(3): 599-617.
There are two subgroups of refractory anemias with greater than five percent myeloblasts: (1) RA with excess blasts (RAEB), defined as 6-20% myeloblasts, and (2) RAEB in transformation (RAEB-T), with 21-30% myeloblasts. The higher the percentage of myeloblasts, the shorter the clinical course and the closer the disease is to acute myelogenous leukemia. Patient transition from early to more advanced stages indicates that these subtypes are merely stages of disease rather than distinct entities. Elderly patients with MDS with trilineage dysplasia and greater than 30% myeloblasts who progress to acute leukemia are often considered to have a poor prognosis because their response rate to chemotherapy is lower than de novo acute myeloid leukemia patients. The recent World Health Organization (WHO) classification (1999) proposes to include all cases of RAEB-T, or patients with greater than 20% myeloblasts, in the category of acute leukemia because these patients have similar prognostic outcomes. However, their response to therapy is worse than the de novo or more typical acute myelogenous leukemia or acute nonlymphocytic leukemia (ANLL) patient. Id.
The fifth type of MDS, the most difficult to classify, is called chronic myelomonocytic leukemia (CMML). This subtype can have any percentage of myeloblasts but presents with a monocytosis of 1000/dL or more. It may be associated with splenomegaly. This subtype overlaps with a myeloproliferative disorder and may have an intermediate clinical course. It is differentiated from the classic chronic myelocytic leukemia (CML) that is characterized by a negative Ph chromosome. The recent WHO classification (1999) proposes that juvenile and proliferative CMML be listed separately from FAB under MDS/myeloproliferative disorder (MPD) with splenomegaly and greater than 13,000 total WBC. CMML is limited to monocytosis, less than 13,000/mm3 total leukocytes, and requires trilineage dysplasia. Id. Harris N. L., et al., J. Clin. Oncol. 1999 December, 17(12): 3835-49. Finally, some other international organizations, including WHO, have suggested a sixth class of MDS patients, characterized by a del (5q) abnormality.
MDS is primarily a disease of elderly people, with the median onset in the seventh decade of life. The median age of these patients is 65 years, with ages ranging from the early third decade of life to as old as 80 years or older. The syndrome may occur in any age group, including the pediatric population. Patients who survive malignancy treatment with alkylating agents, with or without radiotherapy, have a high incidence of developing MDS or secondary acute leukemia. About 60-70% of patients do not have an obvious exposure or cause for MDS, and are classified as primary MDS patients.
The most common cases of MDS are primary, or idiopathic. However, a nonspecific history of exposure to indeterminable chemicals or radiation 10-15 years prior to onset of disease may be present in about 50% of patients. This relationship to pathogenesis remains unproved. Compounds such as, but not limited to, benzene, insecticides, weed killers, and fungicides are possible causes of MDS. Goldberg H., et al., Cancer Res. 1990 Nov. 1; 50(21): 6876-81. Secondary MDS describes development of MDS or acute leukemia after known exposures to chemotherapy drugs that can cause bone marrow damage. These drugs are associated with a high incidence of chromosomal abnormalities following exposure and at the time of MDS or acute leukemia diagnosis.
Further, MDS is associated with complications associated with severe cytopenias. Other complications are development of myelofibrosis, which can accelerate decline in blood counts and increase transfusion requirements. Transformation to acute leukemia accelerates the development of complications such as anemia, bleeding, and infections.
Recently, the International MDS Risk Analysis (IMRA) Workshop proposed an International Prognosis Scoring System (IPSS) to decrease imprecision in predicting survival and AML risk in MDS patients. The IPSS is based on the number of cytopenias, percentage of BM blasts, and type of cytogenetic abnormalities (Table 1). Greenberg P, Cox C, Le Beau M M, et al., Blood 1997, 89:2079-88. The latter are categorized into good (normal, −Y, del (5q), del (20q)), intermediate, and poor subgroups (complex or chromosome 7 abnormalities).
TABLE 1International Prognostic Scoring System for MDSScore ValuePrognostic00.51.01.52.0VariableBone marrow<55-10—11-2021-30blasts (%)Karyotype*GoodIntermediatePoorCytopenias0/12/3*Good, normal, del (5q), del (20q), -Y; Poor, complex (>3) or chromosome 7 abnormalities; Intermediate, +8, and other single or double abnormalities.
2.2 MDS Treatment
The current treatment of MDS is based on the stage and the mechanism of the disease that predominates the particular phase of the disease process. Bone marrow transplantation has been used in patients with poor prognosis or late-stage MDS. Epstein and Slease, 1985, Surg. Ann. 17:125. This type of therapy, however, is both painful for donor and recipient, because of the involvement of invasive procedures and can cause severe and even fatal complications to the recipient, particularly with allogeneic transplant and related Graft Versus Host Disease (GVHD) results. Therefore, the risk of GVHD restricts the use of bone marrow transplantation to patients with otherwise fatal diseases. Further, as most patients are elderly and only a few young MDS patients will have a matched donor, the use of bone marrow transplantation is limited.
An alternative approach to therapy for MDS is the use of hematopoietic growth factors or cytokines to stimulate blood cell development in a recipient. Dexter, 1987, J. Cell Sci. 88:1; Moore, 1991, Annu. Rev. Immunol. 9:159; and Besa E. C., Med. Clin. North Am. 1992 May, 76(3): 599-617. The process of blood cell formation, by which a small number of self-renewing stem cells give rise to lineage specific progenitor cells that subsequently undergo proliferation and differentiation to produce the mature circulating blood cells has been shown to be at least in part regulated by specific hormones. These hormones are collectively known as hematopoietic growth factors. Metcalf, 1985, Science 229:16; Dexter, 1987, J. Cell Sci. 88:1; Golde and Gasson, 1988, Scientific American, July:62; Tabbara and Robinson, 1991, Anti-Cancer Res. 11:81; Ogawa, 1989, Environ. Health Presp. 80:199; and Dexter, 1989, Br. Med. Bull. 45:337. The most well characterized growth factors include erythropoietin (EPO), granulocyte macrophage colony stimulating factor (GM-CSF), and granulocyte colony stimulating factor (G-CSF). Apart from inducing proliferation and differentiation of hematopoietic progenitor cells, such cytokines have also been shown to activate a number of functions of mature blood cells, including influencing the migration of mature hematopoietic cells. Stanley et al., 1976, J. Exp. Med. 143:631; Schrader et al., 1981, Proc. Natl. Acad. Sci. U.S.A. 78:323; Moore et al., 1980, J Immunol. 125:1302; Kurland et al., 1979, Proc. Natl. Acad. Sci. U.S.A. 76:2326; Handman and Burgess, 1979, J. Immunol. 122:1134; Vadas et al., 1983, Blood 61:1232; Vadas et al., 1983, J. Immunol. 130:795; and Weibart et al., 1986, J. Immunol. 137:3584.
Unfortunately, hematopoietic growth factors have not proven effective in many clinical settings. Clinical trials of MDS patients treated with recombinant human GM-CSF and G-CSF have shown that while these cytokines can restore granulocytopoiesis in treated patients, their efficacy is restricted to the granulocyte or monocyte lineage with little or no improvement in hemoglobin or platelet counts. Schuster et al., 1990, Blood 76 (Suppl.1):318a. When such patients were treated with recombinant human EPO, a sustained improvement in hemoglobin or decrease in transfusion requirement was achieved in only less than 25% of patients. Besa et al., 1990, 76 (Suppl.1):133a; Helistrom et al., 1990, 76 (Suppl.1):279a; Bowen et al., 1991, Br. J Haematol. 77:419. Therefore, there remains a need for safe and effective methods of treating and managing MDS.
2.3 Selective Cytokine Inhibitory Drugs
Compounds referred to as SelCIDs™ (Celgene Corporation) or Selective Cytoline Inhibitory Drugs have been synthesized and tested. These compounds potently inhibit TNF-α production, but exhibit modest inhibitory effects on LPS induced IL1β and IL12, and do not inhibit IL6 even at high drug concentrations. In addition, SelCIDs™ tend to produce a modest IL10 stimulation. L. G. Corral, et al., Ann. Rheum. Dis. 58:(Suppl I) 1107-1113 (1999).
Further characterization of the selective cytokine inhibitory drugs shows that they are potent PDE4 inhibitors. PDE4 is one of the major phosphodiesterase isoenzymes found in human myeloid and lymphoid lineage cells. The enzyme plays a crucial part in regulating cellular activity by degrading the ubiquitous second messenger cAMP and maintaining it at low intracellular levels. Id. Inhibition of PDE4 activity results in increased cAMP levels leading to the modulation of LPS induced cytokines including inhibition of TNF-α production in monocytes as well as in lymphocytes.